1. Field of the Invention
The present invention relates to an optical fiber for an optical fiber laser, a method for fabricating the same, and an optical fiber laser, in more particular, to an optical fiber for a high output power optical fiber laser having a rare earth element doped core and a cladding, a method for fabricating the same, and an optical fiber laser.
Development of a less expensive light source with a higher output power has been required for laser processing, medical application, and the like. For these requirements, an optical fiber laser has been noted since it is possible to easily extract a laser light with high efficiency and high quality.
FIG. 9 shows an optical fiber 91 to be used for a high output power optical fiber laser. This optical fiber 91 is a double clad fiber comprising a core 92 doped with rare earth element such as Yb, Er, Er/Yb, Tm, Nd, and a cladding 93 comprising a first cladding 93a and a second cladding 93b. A coating layer (not shown) comprising UV curing resin or the like is provided at an outer periphery of the second cladding 93b. 
A light outputted from a multimode laser diode (semiconductor laser) is input into an end (input end) of the optical fiber 91 as an exciting light Le9. The exciting light Le9 collected into the first cladding 93a is propagated though the optical fiber 91, thereby exciting the rare earth elements in the core 92. Then, the exciting light is propagated to the core 92 from the excited rare earth elements so that a high output power laser exciting light L9 is output from another end (output end) of the optical fiber 91.
2. Related Art
For example, Japanese Patent Laid-Open No. 5-249328 (JP-A-5-249328) discloses an example of the conventional optical fiber for an optical fiber laser having a rare earth element doped core divided into a plurality of core regions, in which regions doped with rare earth element in the respective core regions have different diameters.
However, when a Yb doped core is used as the core 92 in the conventional optical fiber 91, an optical conversion efficiency of the laser exciting light L9 to the exciting light Le9 is about 80%, and about 20% of energy is converted into heat.
Accordingly, in the conventional optical fiber 91, there is a disadvantage in that a temperature of the optical fiber 91 is increased in accordance with an increase in the output power of the optical fiber laser. In particular, since a temperature elevation of a part in vicinity of the input end of the optical fiber 91 is large, the coating layer of the optical fiber 91 may be damaged, so that the output of the optical fiber laser is limited.
In addition, there are limiting factors for the high output power optical fiber laser, such as non-linearity of the optical fiber, damage of the optical fiber, excitation method. Namely, in the conventional optical fiber, higher exciting light power is required in accordance with the increase in the output power of the optical fiber laser, so that an energy density in the double clad fiber is excessive, thereby causing the problems such as the heat generation in the optical fiber, the non-linearity of the optical fiber, the damage or breakdown of the optical fiber.
In actually commercialized optical fiber laser, it is required to remove the aforementioned limiting factors with a good balance.